1610s, "to identify as a bastard," from bastard (q.v.) + -ize. The figurative sense, "to make degenerate, debase" is earlier (1580s), probably because bastard also was serving as a verb meaning "to declare illegitimate" (1540s). Related: Bastardized; bastardizing; bastardization.
[bastardize etymology, bastardize origin, 英语词源]